What is PCB design?
The design of the printed circuit board is based on the circuit schematic diagram to achieve the functions required by the circuit designer. The design of printed circuit boards mainly refers to the layout design, which needs to consider the layout of external connections. The optimal layout of internal electronic components, the optimal layout of metal wiring and through holes, electromagnetic protection, heat dissipation and other factors. Excellent layout design can save production costs and achieve good circuit performance and heat dissipation performance. Simple layout design can be realized by hand, and complex layout design needs to be realized by computer aided design (CAD).
Design software
- protel, protel 99se, protel DXP, Altium are used more in China, which are all software developed by a company and constantly upgraded; The current version is Altium Designer 15, which is relatively simple and the design is more casual, but it is not very good to do complex PCB software.Cadence spb software Cadence spb This is Cadence software, the current version is Cadence SPB 16.5; The ORCAD schematic design is the international standard; Among them, PCB design and simulation are very complete, which is more complex than protel, mainly due to complex requirements and Settings; However, the provisions for the design are made, so the design gets twice the result with half the effort, which is obviously stronger than protel.
- Mentor’s BORDSTATIONG and EE, among which BOARDSTATION is only applicable to UNIX system, not designed for PC, so fewer people use it; The current MentorEE version is MentorEE 7.9 and Cadence spb belong to the same level of PCB design software, it is worse than cadence spb in some places, its strengths are pulling wire, flying wire, known as flying wire king.
- EAGLE Layout is the most widely used PCB design software in Europe. The PCB design software mentioned above is used more, Cadence spb and MentorEE are well-deserved Kings inside. If it is a beginner to design PCB, I think Cadence spb is better, it can give designers a good design habit, and can ensure good design quality.
Setup technique
Different points need to be set up in different stages of design, and large grid points can be used for device layout in the layout stage.
For large devices such as ics and non-positioning connectors, the grid accuracy of 50 to 100mil can be selected for layout, and for passive small devices such as resistance capacitors and inductors, 25mil grid can be used for layout. The accuracy of large grid points is conducive to the device alignment and beautiful layout.
PCB layout rules:
- under normal circumstances, all the components should be arranged on the same surface of the circuit board, only when the top component is too dense, in order to put some highly limited and small heat devices, such as chip resistance, chip capacitor, chip IC, etc., on the bottom.
- under the premise of ensuring electrical performance, components should be placed on the grid and parallel to each other or vertical arrangement, in order to neat, beautiful, under normal circumstances do not allow components overlap; The arrangement of components should be compact, and the components should be evenly distributed and dense throughout the layout.
- the minimum spacing between different components on the circuit board and the welding pad graphics should be more than 1MM.
- the edge of the circuit board is generally not less than 2MM. The best shape of the circuit board is rectangular, and the aspect ratio is 3:2 or 4:3. When the surface of the circuit board is greater than 200MM by 150MM, the mechanical strength that the circuit board can withstand should be considered.
Layout technique
In the layout design of the PCB to analyze the unit of the circuit board, according to the function of the layout design, the layout of all the components of the circuit, to meet the following principles:
- arrange the position of each functional circuit unit according to the process of the circuit, so that the layout is convenient for signal flow, and keep the signal in the same direction as possible.
- Take the core components of each functional unit as the center, and carry out the layout around him. The components should be evenly, integral and compact arranged on the PCB to minimize and shorten the leads and connections between the components.
- the circuit operating at high frequency should consider the distribution parameters between components. The general circuit should be as far as possible so that the components are arranged in parallel, so that it is not only beautiful, but also easy to install and weld, and easy to mass production.
Design procedure
Layout design
In the PCB, special components refer to the key components of the high-frequency part, the core components in the circuit, the components that are susceptible to interference, the components with high pressure, the components with large heat, and some heterosexual elements, the location of these special components need to be carefully analyzed, and the layout of the belt meets the requirements of the circuit function and the production needs. Improper placement of them can create circuit compatibility issues, signal integrity issues, and lead to PCB design failure.
PCB size is considered first when designing how to place special components. Q&e pointed out that when the pcb size is too large, the printing line is long, the impedance is increased, the anti-dryness ability is decreased, and the cost is also increased; After hours, the heat dissipation is not good, and the adjacent lines are easy to be disturbed. After determining the size of the PCB, the pendulum position of the special component is determined. Finally, according to the functional unit, all the components of the circuit are laid out. The location of special components in the layout should generally comply with the following principles:
- shorten the connection between high-frequency components as much as possible, and try to reduce their distribution parameters and electromagnetic interference between each other. The components that are susceptible to interference should not be too close to each other, and the input and output should be as far away as possible.
- Some components or wires may have a high potential difference, and their distance should be increased to avoid accidental short circuit caused by discharge. High-voltage components should be placed out of reach of hands as far as possible.
- the weight of more than 15G components, can be fixed by the support, and then welded. Those heavy and hot components should not be placed on the circuit board, should be placed on the bottom of the main box, and consider the heat dissipation problem. Heat-sensitive components should be kept away from heat-generating components.
- the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, micro-switches should consider the structural requirements of the whole board, and some frequently used switches should be placed in a place that is easily accessible to the hand if the structure allows. The layout of components is balanced, dense and can not be top-heavy.
The success of a product, one is to pay attention to internal quality. The second is to take into account the overall beauty, both are more perfect board, in order to become a successful product.
Placement sequence
- Place components that are closely matched with the structure, such as power sockets, indicators, switches, connectors, etc.
- place special components, such as large components, heavy components, heating components, transformers, ics, etc.
- Place small components.
Layout check
- the size of the circuit board and the drawing requirements of the processing size is consistent.
- Whether the layout of the components is balanced, arranged neatly, and whether it has been fully deployed.
- There is no conflict at all levels. For example, the components, the outer frame, and the level that requires screen printing are reasonable.
- the commonly used components are convenient to use. Such as switches, plug-in board insertion equipment, components that need to be replaced frequently.
- Whether the distance between thermal components and heating components is reasonable.
- whether the heat dissipation is good.
- whether the interference problem of the line needs to be considered
Circuit schematic diagram
The design of the circuit schematic is the basis of the entire circuit design, and its design directly determines the effect of the PCB design behind it. In general, the circuit schematic is designed as follows.
Specific design steps:
- launch the ProtelDXP schematic editor
- The second step is to set the size and layout of the circuit schematic
- remove the required component from the component library and place it on the working plane
- Connect components according to design needs
- adjust the components after wiring
- save the schematics document that has been drawn
- Print out the drawings
Main process
In PCB design, in fact, before the formal wiring, but also through a very long step, the following is the main process of PCB design:
System specification
First of all, we must plan out the system specifications of the electronic equipment. This includes system functionality, cost constraints, size, operation, and so on.
Function block
Next you have to make a functional block diagram of the system. The relationships between blocks must also be marked.
If the system is divided into several PCBS, it can not only be reduced in size, but also make the system have the ability to upgrade and exchange parts. The system function block diagram provides the basis for our segmentation. For example, a computer can be divided into a host board, a video card, a sound card, a floppy disk drive, and a power supply.
Decide to use the packaging method, and the size of each PCB When the technology and the number of circuits used by each PCB are decided, the next is to determine the size of the board. If the PCB design is too large, then the packaging technology must be changed, or re-segmentation action. When choosing technology, the quality and speed of the diagram should also be taken into account.
High speed process
The traditional PCB design process is no longer applicable in the field of high-speed PCB design with increasingly high signal rates and even above GHZ. High-speed PCB design must be perfectly integrated with simulation and validation. Simulation is not the simple verification of the design in the traditional sense, but the pre-simulation embedded in the entire design process to obtain rules, rules drive the design, and finally the post-simulation verification.
Matters needing attention
(1) Avoid arranging important signal lines at the edge of the PCB, such as clocks and reset signals.
(2) The distance between the ground wire and the signal line is at least 4 mm; Keep the aspect ratio of the housing ground wire less than 5:1 to reduce inductance effects.
(3) The devices and wires that have been located are locked with the LOCK function so that they are not moved by mistake in the future.
(4) The minimum width of the wire should not be less than 0.2mm (8mil), in the high-density and high-precision printed line, the wire width and spacing are generally desirable 12mil.
(5) The principle of 10-10 and 12-12 can be applied to the wiring between the DIP-packaged IC feet, that is, when two wires pass between the two feet, the diameter of the pad can be set to 50mil, the line width and line distance are 10mil, when only one wire passes between the two feet, the diameter of the pad can be set to 64mil, the line width and line distance are 12mil.
(6) When the diameter of the pad is 1.5mm, in order to increase the stripping strength of the pad, the length of the pad is not less than 1.5mm, the width is 1.5mm and the oblong pad can be used.
(7) When the wiring of the welding pad is thin, the connection between the welding pad and the wiring should be designed into a droplet shape, so that the welding pad is not easy to peel, and the wiring and the welding pad are not easy to disconnect.
(8) In the design of large-area copper coating, there should be an open window on the copper coating, plus a heat dissipation hole, and the opening window is designed into a network.
(9) Shorten the connection between high-frequency components as much as possible to reduce their distribution parameters and electromagnetic interference between each other. Components that are susceptible to interference should not be too close to each other, and input and output components should be as far away as possible.
Special element
1, high-frequency components: the shorter the connection between high-frequency components, the better, try to reduce the distribution parameters of the connection and electromagnetic interference between each other, susceptible to interference components can not be too close. The distance between the components belonging to the input and the components belonging to the output should be as large as possible.
(2) Components with high potential difference: the distance between components with high potential difference and the connection should be increased to avoid damage to components when there is an accidental short circuit. In order to avoid the occurrence of creepage, it is generally required that the distance between the copper film line between 2000V potential difference should be greater than 2mm, and if the potential difference is higher, the distance should be increased. Devices with high voltage should be arranged as far as possible in places that are difficult to reach by the hand during debugging.
3, the weight is too large components: such components should have a bracket fixed, and for large and heavy, heat generating components, should not be installed on the circuit board.
4, heating and thermal components: pay attention to heating components should be far away from thermal components.
Regular assembly
In the design, from the perspective of the assembly of the PCB board, the following parameters should be considered:
(1) The diameter of the hole should be determined according to the maximum material condition (MMC) and the minimum material condition (LMC). The diameter of the hole of an unsupported component should be chosen by subtracting the MMC of the pin from the MMC of the hole, resulting in a difference between 0.15-0.5 mm. And for ribbon pins, the difference in bore diameter between the nominal diagonal of the pin and the unsupported hole will be no more than 0.5mm and no less than 0.15mm.
(2) Reasonably place smaller components so that they will not be covered by larger components.
(3) The thickness of solder resistance should not be greater than 0.05mm.
(4) Screen printing mark can not be intersected with any pad.
(5) The top half of the circuit board should be the same as the bottom half to achieve structural symmetry. Because asymmetrical boards can bend.
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